1. Field of the Invention
The present invention relates to a zoom lens unit and a method of driving the same for driving lenses using an electrostatic actuator, and more particularly, to a zoom lens unit and a method of driving the same capable of separately controlling a plurality of movable parts.
2. Description of the Related Art
In recent years, assembling a camera unit having a zoom function in mobile equipment such as mobile phones has been examined. In such a camera unit, the focal point is adjusted by driving lenses, and an image is finally formed on a sensor. Electrostatic actuators may be used as a drive source for driving the lenses along the optical axis.
The zoom lens unit adjusts the zoom magnification by driving a plurality of lenses. The electrostatic actuator includes, for example, a stationary part, a first movable part, and a second movable part, and each of the first and second movable parts holds a lens.
The stationary part includes a driving electrode substrate and a holding electrode substrate attached to the upper and lower inner walls of a stationary part frame in FIG. 1. Further, the first and second movable parts are disposed such that they can be reciprocated in the axial direction of the lenses with a gap of several microns between the pair of electrode substrates.
In the zoom lens unit configured as described above, the first and second movable parts can be driven by an electrostatic force by supplying a voltage to the electrodes of the pair of electrode substrates of the stationary part in a predetermined sequence using a switching circuit.
The zoom lens unit described above has the following problems. That is, when the common electrode substrates are used with respect to the plurality of movable parts, the plurality of movable parts can be driven only separately, respectively, because one of the movable parts is driven by supplying the voltage to the driving electrode substrate in the predetermined sequence while holding the other of the movable parts by the holding electrode substrate.
In the zoom lens unit, the respective lenses must trace a zoom curve based on predetermined lens design to vary the zoom magnification. When the zoom curve is traced, it is not preferable to separately drive the respective groups of lenses. This is because when the lenses are driven separately, the zoom magnification is not continuously varied at a constant speed and is changed intermittently, and the user gets the impression that the image is irregularly output onto a screen and it is difficult to view the image.
When, for example, a second group of lenses acts to vary the zoom magnification and a first group of lenses acts to adjust the focal point, the zoom curve is traced in such a sequence that the magnification is varied by moving the second group of lenses first and then focusing is executed by driving the first group of lenses, thereby the magnification is varied intermittently.
To prevent the above problem, the plurality of movable parts must be simultaneously driven in the same direction or in an opposite direction. However, to drive the plurality of movable parts independently, as many stationary parts as movable parts are required, which increases the volume of an actuator unit with an increase in its size. Note that there is a configuration by which the plurality of movable parts are driven independently by devising the disposition of the electrodes of the stationary part. In the configuration, however, a driving force may be in short supply.
Further, when a plurality of movable parts are provided in other drive systems (for example, an electromagnetic device and a piezoelectric device), as many stationary parts as the movable parts are necessary, thereby the volume of the actuator unit is increased with an increase in its size.
In contrast, when the plurality of movable parts are simultaneously driven using a cam mechanism and the like, it is difficult to drive the movable parts separately. Thus, it is difficult to adjust the focal point and to cope with a change of the focal point due to a change of temperature in an external environment which are required to a lens unit. In this case, a significant burden is placed on the selection of a lens material and on the optical design of lenses.